The present invention relates to a heater and, more particularly, to a compact high-efficient air heater.
Prior art air heaters typically include both a heating section and a blower unit, which blows the cool air to be heated through the heating section and thereafter distributes the heated air into a room. The heating section includes both a combustion chamber in which a combustible fuel is burned to produce hot combustion product gases (commonly referred to as flue gases), and a heat exchanger through which the flue gases are directed. The cool air to be heated is simultaneously directed through the heat exchanger whereby the heat from the hot flue gases is transferred via the heat exchanger to the cool air. The heated air is then directed into the room. Thus, the mentioned heater not only heats the air, but also distributes the heated air into the room.
This same air heater is typically utilized as a "stand alone" unit, and is often mounted at an elevated location, e.g., it may be suspended from the ceiling of a room. Thus, the heated air exiting the heater must not only be blown out from the heater, but must also be directed downwards into the room. It will be recognized that stratification (i.e., the tendency of warm air to rise within a room) is a condition which must be specifically addressed, particularly since the heater may already be positioned at an elevated location.
In certain prior art applications, one or more ceiling fans may be installed in the room to move the heated air downward and reduce stratification. These ceiling fans are typically controlled by a thermostat independent of the heater. Thus, in the past, the condition of stratification has been addressed independently of the operation of the heater often leading to inefficient operation of the heater and uncomfortable temperature swings.
Next, typical prior art heaters utilize cross-flow heat exchangers wherein the flue gases are directed through a first set of passages in the heat exchanger, while the cool air to be heated is directed crosswise through a second set of passages in the heat exchanger which alternate with the first set of passages whereby the heat from the hot flue gases is transferred through the metal components of the heat exchanger to the cool air. It will be appreciated that the mass flow of cool air through the heat exchanger of an air heater is significantly greater than the mass flow of hot flue gases through the same heat exchanger. The ratio of the mass flow of the cool air to the flue gases typically ranges from 25:1 to 250:1. To facilitate this unequal flow of fluids, typically prior art heat exchangers require the necessity of a larger blower unit (which translates into higher energy costs) and/or require an increase in the size of the heat exchanger (thus increasing the overall size of the heater).
It will be recognized by those skilled in the art that prior art heaters utilizing only a single stage heat exchanger typically exhibit 78 to 80% thermal efficiency. When higher efficiency is desired, prior art heaters must add a second heat exchanger, which further cools the flue gases (typically to a point below the dew point) whereby a thermal efficiency of 90% plus can be achieved. The addition of a second heat exchanger, however, increases the cost and/or the overall size of the heater.
It will also be appreciated that prior art cross-flow heat exchangers typically exhibit an unwanted "temperature gradient" along the air outlet side of the heat exchanger. Stated differently, the surface of the heat exchanger through which the heated air exits will exhibit a temperature gradient wherein the portion of such surface closest to the flue gas inlet is significantly hotter than the portion of such surface proximate the flue gas outlet. Because the air exiting the heat exchanger exhibits this temperature gradient, heating a room becomes more difficult and the comfort of the persons working within the room may be affected.
It will be further appreciated that the portion of the heat exchanger proximate the combustion chamber is exposed to very high temperatures. Prior art heaters typically address the exposure to these high temperatures through the use of high temperature resistant materials, insulation, and/or certain design criteria, e.g., increased size of the unit.
Those skilled in the art will also recognize that condensation occurs within the flue gas passages during the heat transfer process, and that this condensation must be removed from the heat exchanger. It will be further appreciated by those skilled in the art that different heating environments may require significantly different sized heaters. Prior art heaters are not typically designed to be modular in concept, and thus are not readily adapted to such different heating requirements.
There is therefore a need in the art for a compact air heater which exhibits a high efficiency (preferably through cooling of the flue gases below the dew point) even in a one stage construction, provides for ready removal of condensation from the flue gas passages, facilitates unequal mass flows of fluids through the heat exchanger without requiring an increase in overall size of the heat exchanger and/or an increase in the size of the fan/blower, reduces the temperature gradient along the air outlet side of the heat exchanger, reduces the tendency of the air in the room to stratify, provides improved comfort regulation, and provides a design which is modular in concept thus allowing the heater unit to be more specifically designed for a particular heating application.